Quantification of turbulence properties in bubble plumes using vortex identification methods

Abstract

Turbulence characteristics within bubble plumes are quantified in the laboratory using particle image velocimetry (PIV) data and postprocessing techniques to identify continuous phase vortices and their properties. Experiments are conducted using a single, high-speed Phantom camera (frame rate 250 Hz); images are preprocessed to eliminate signatures from the bubbles, resulting in high temporal and spatial resolution PIV data for the continuous phase only. The local swirl strength is used to identify vortices in the flow field. By combining the velocity fields and identified vortex maps, the vortex size probability distribution and the relationship of the vortex size, circulation and enstrophy with respect to the position in the plume are quantified. Results demonstrate that these quantities are self-similar when nondimensionalized by the plume radius and center line velocity. A direct relationship is also identified between vortex size and the corresponding average circulation and enstrophy. Modulation of the turbulent energy spectrum compared to a single-phase plume is noted and is attributed to the presence of the bubbles in the flow. Vortex size and energy density are greatest in the shear layers at the edge of the round plume, indicating that shear instability, and not bubble wakes, is the dominant process of large-scale turbulent and coherent structure generation in bubble plumes.